Biasing of electromechanical systems transducer with alternating-current voltage waveform

ABSTRACT

A MEMS microphone may include a backplate comprising first and second electrodes electrically isolated from one another and mechanically coupled to the backplate in a fixed relationship relative to the backplate and a diaphragm configured to displace relative to the backplate as a function of sound pressure incident upon the diaphragm, the diaphragm comprising third and fourth electrodes electrically isolated from one another and mechanically coupled to the diaphragm in a fixed relationship relative to the diaphragm such that the third and fourth electrodes displace relative to the backplate as a function of sound pressure incident upon the diaphragm. The first and third electrodes may form a first capacitor and the second and fourth electrodes may form a second capacitor, the capacitance of each which may be a function of the displacement of the diaphragm, and each of which may be biased by an alternating-current voltage waveform.

RELATED APPLICATIONS

The present disclosure is related to U.S. Provisional Patent ApplicationSer. No. 62/438,144, filed Dec. 22, 2016, which is incorporated byreference herein in its entirety.

FIELD OF DISCLOSURE

The present disclosure relates in general to audio systems, and moreparticularly, to improving the performance of microelectromechanicalsystems (MEMS) based transducers as compared to traditional approaches.

BACKGROUND

Microphones are ubiquitous on many devices used by individuals,including computers, tablets, smart phones, and many other consumerdevices. Generally speaking, a microphone is an electroacoustictransducer that produces an electrical signal in response to deflectionof a portion (e.g., a membrane or other structure) of a microphonecaused by sound incident upon the microphone. For example, a microphonemay be implemented as a MEMS transducer. A MEMS transducer may include adiaphragm or membrane having an electrical capacitance to a referenceplane or backplate, such that a change in acoustic pressure applied tothe MEMS transducer causes a deflection or other movement of themembrane, and thus causes a change in the electrical capacitance. Suchelectrical capacitance or the change thereof may be sensed by a sensingcircuit and processed.

Existing MEMS microphone implementations are susceptible to a variety ofelectronic noise sources that may negatively affect a signal-to-noiseratio of a MEMS microphone. Among such noise sources are flicker noise(also known as 1/f noise) and noise bias resistors used in the biasingtopology of existing approaches. Also, as with any audio system, it maybe desirable to improve dynamic range of MEMS microphone implementationscompared to that of existing implementations.

SUMMARY

In accordance with the teachings of the present disclosure, certaindisadvantages and problems associated with existing MEMS transducers maybe reduced or eliminated.

In accordance with embodiments of the present disclosure, amicroelectromechanical systems microphone may include a backplatecomprising a first plurality of electrodes comprising at least a firstelectrode and a second electrode electrically isolated from one anotherand each are mechanically coupled to the backplate in a fixedrelationship relative to the backplate, and a diaphragm configured tomechanically displace relative to the backplate as a function of soundpressure incident upon the diaphragm, wherein the diaphragm comprises asecond plurality of electrodes, the second plurality of electrodescomprising at least a third electrode and a fourth electrode, whereinthe third electrode and the fourth electrode are electrically isolatedfrom one another and each is mechanically coupled to the diaphragm in afixed relationship relative to the diaphragm such that the secondplurality of electrodes mechanically displace relative to the backplateas the function of sound pressure incident upon the diaphragm. The firstelectrode and the third electrode may form a first capacitor having afirst capacitance which is a function of a displacement of the diaphragmrelative to the backplate. The second electrode and the fourth electrodemay form a second capacitor having a second capacitance which is afunction of the displacement of the diaphragm relative to the backplate.Each of the first capacitor and the second capacitor may be biased by analternating-current voltage waveform.

In accordance with these and other embodiments of the presentdisclosure, a method may include biasing a first capacitor of anelectromechanical systems microphone with an alternating-current voltagewaveform and biasing a second capacitor of the electromechanical systemsmicrophone with the alternating-current voltage waveform. The firstcapacitor may be formed from a first electrode and a third electrode andthe second capacitor may be formed from a second electrode and a fourthelectrode. The first electrode and the second electrode may beelectrically isolated from one another and may each be mechanicallycoupled to a backplate in a fixed relationship relative to thebackplate, the backplate comprising a first plurality of electrodescomprising the first electrode and the second electrode. The thirdelectrode and the fourth electrode may be electrically isolated from oneanother and may each be mechanically coupled to a diaphragm in a fixedrelationship relative to the diaphragm such that the second plurality ofelectrodes mechanically displace relative to the backplate as thefunction of sound pressure incident upon the diaphragm whichmechanically displaces the diaphragm relative to the backplate. Thefirst capacitor may have a first capacitance which is a function of adisplacement of the diaphragm relative to the backplate and the secondcapacitor may have a second capacitance which is a function of adisplacement of the diaphragm relative to the backplate.

Technical advantages of the present disclosure may be readily apparentto one having ordinary skill in the art from the figures, descriptionand claims included herein. The objects and advantages of theembodiments will be realized and achieved at least by the elements,features, and combinations particularly pointed out in the claims.

It is to be understood that both the foregoing general description andthe following detailed description are explanatory examples and are notrestrictive of the claims set forth in this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present embodiments and advantagesthereof may be acquired by referring to the following description takenin conjunction with the accompanying drawings, in which like referencenumbers indicate like features, and wherein:

FIG. 1 illustrates a block diagram of selected components of an exampleMEMS microphone, in accordance with embodiments of the presentdisclosure; and

FIG. 2 illustrates a block diagram of selected components of an exampleaudio system comprising a MEMS microphone biased with analternating-current voltage waveform, in accordance with embodiments ofthe present disclosure.

DETAILED DESCRIPTION

FIG. 1 illustrates a block diagram of selected components of an exampleMEMS microphone 100, in accordance with embodiments of the presentdisclosure. As shown in FIG. 1, MEMS microphone 100 may comprise abackplate 102, a diaphragm 108, and a substrate 114.

Backplate 102 may include a plurality of electrodes comprising at leasta first electrode 104 and a second electrode 106 electrically isolatedfrom one another and each mechanically coupled to backplate 102 in afixed relationship relative to backplate 102. For the purposes ofclarity and exposition, only two electrodes 104 and 106 are depicted inFIG. 1. In some embodiments, backplate 102 may include more than twoelectrodes.

Diaphragm 108 may comprise a membrane or other structure configured tomechanically displace relative to the backplate as a function of soundpressure incident upon diaphragm (e.g., through acoustic port 118 ofsubstrate 114). As shown in FIG. 1, backplate 102 may be mechanicallycoupled to diaphragm 108 via a plurality of support posts 116 ofbackplate 102. Diaphragm 108 may include a plurality of electrodescomprising at least a third electrode 110 and a fourth electrode 112, inwhich third electrode 110 and fourth electrode 112 are electricallyisolated from one another and each is mechanically coupled to diaphragm108 in a fixed relationship relative to diaphragm 108 such thatelectrodes 110 and 112 mechanically displace relative to backplate 102as the function of sound pressure incident upon diaphragm 108.

Substrate 114 may comprise any suitable substrate or surface (e.g., asemiconductor substrate) upon which MEMS microphone 100 may befabricated. The various components of MEMS microphone 100 (e.g.,backplate 102, electrode 104, electrode 106, diaphragm 108, electrode110, electrode 112, support posts 116, etc.) may be formed on substrateusing semiconductor fabrication techniques now known or semiconductorfabrication techniques that may be known in the future. As also shown inFIG. 1, substrate 114 may also have an acoustic port 118 formed therein(e.g., using semiconductor fabrication techniques now known orsemiconductor fabrication techniques that may be known in the future)through which sound pressure may propagate to diaphragm 108 to displacediaphragm 108 as a function of such sound pressure.

MEMS microphone 100 may be constructed such that first electrode 104 andthird electrode 110 electrically interact with one another so as to forma first capacitor having a first capacitance which is a function of adisplacement of diaphragm 108 relative to backplate 102. Similarly, MEMSmicrophone 100 may be constructed such that second electrode 106 andfourth electrode 112 electrically interact with one another so as toform a second capacitor having a second capacitance which is a functionof a displacement of diaphragm 108 relative to backplate 102.

The implementation shown in FIG. 1 may be one of many ways to constructa MEMS microphone in accordance with the present disclosure.Accordingly, one or more other implementations may exist for a MEMSmicrophone which are substantially equivalent to that of MEMS microphone100 depicted in FIG. 1.

FIG. 2 illustrates a block diagram of selected components of an exampleaudio system 200 comprising a MEMS microphone biased with analternating-current voltage waveform, in accordance with embodiments ofthe present disclosure. As shown in FIG. 2, audio system 200 maycomprise a voltage supply 202, first capacitor 204 formed from firstelectrode 104 and third electrode 110, second capacitor 206 formed fromsecond electrode 106 and fourth electrode 112, a third capacitor 208, afourth capacitor 210, an amplifier 212, a demodulator 214, ananalog-to-digital converter (ADC) 216, a direct-current voltage detector218, a peak detector 220, a dynamic range enhancement controller 222, acapacitance measurement subsystem 224, a gain controller 226, and a gainelement 228.

Voltage supply 202 may comprise any suitable system, device, orapparatus configured to output an alternating-current (AC) bias voltageV_(BIAS) for biasing first capacitor 204 and second capacitor 206, asdescribed in greater detail below. In some embodiments, voltage supply202 may generate AC bias voltage V_(BIAS) as a square-wave voltagewaveform. However, any suitable AC waveform may be used. Voltage supply202 may be implemented in any suitable manner, including withoutlimitation with a charge pump power supply. In some embodiments, AC biasvoltage V_(BIAS) may have a frequency greater than that of human hearing(e.g., greater than 20 kilohertz).

As shown in FIG. 2, first electrode 104 of first capacitor 204 may beelectrically coupled to a first terminal of voltage supply 202 andsecond electrode 106 of second capacitor 206 may be electrically coupledto a second terminal of voltage supply 202. Accordingly, each of firstcapacitor 204 and second capacitor 206 may be biased by thealternating-current voltage waveform generated by voltage supply 202.Furthermore, as shown in FIG. 2, first capacitor 204 and secondcapacitor 206 may be electrically coupled to one another in a bridgestructure, the bridge structure comprising third capacitor 208 in serieswith first capacitor 204 and coupled between first capacitor 204 and thesecond terminal of voltage supply 202 and fourth capacitor 210 in serieswith second capacitor 206 and coupled between second capacitor 206 andthe first terminal of voltage supply 202.

In operation, a differential signal V_(CAP) comprising the difference inpotential between a third electrode 110 and fourth electrode 112 may begenerated due to the presence of AC bias voltage V_(BIAS) and soundpressure incident on diaphragm 108 which induces changes in capacitancesof first capacitor 204 and second capacitor 206. Thus, differentialsignal V_(CAP) may comprise an audio signal which is a function of thesound pressure incident on diaphragm 108, wherein such audio signal ismodulated at a frequency of AC bias voltage V_(BIAS).

Amplifier 212 may comprise any suitable system, device, or apparatusconfigured to amplify an analog signal received at its input (e.g.,differential signal V_(CAP)) to an amplified version of the input analogsignal which may be more suitable for downstream processing.

Demodulator 214 may comprise any suitable system, device, or apparatusconfigured to extract from an analog signal (e.g., differential signalV_(CAP) as amplified by amplifier 212) an information-bearing signal(e.g., analog microphone signal V_(MIC)) from a modulated carrier wave(e.g., a modulated carrier wave at the frequency of AC bias voltageV_(BIAS)). In some embodiments, demodulator 214 may comprise asynchronous modulator.

ADC 216 may comprise any suitable system, device, or apparatusconfigured to convert an analog signal (e.g., analog microphone signalV_(MIC)) into a corresponding digital signal (e.g., digital microphonesignal MIC_DIG).

Direct-current voltage detector 218 may comprise any suitable system,device, or apparatus configured to receive digital microphone signalMIC_DIG and analyze such signal in order to detect a direct-currentvoltage offset present in differential signal V_(CAP) of the bridgestructure comprising capacitors 204, 206, 208, and 210. Based on thedetected direct-current voltage offset present in differential signalV_(CAP), direct-current voltage detector 218 may generate adirect-current compensation control signal DC_COMP to control at leastone of the third capacitance of third capacitor 208 and the fourthcapacitance of fourth capacitor 210 responsive to the direct-currentvoltage offset in order to compensate for the presence of thedirect-current voltage offset.

Peak detector 220 may include any suitable system, device, or apparatusconfigured to receive analog microphone signal V_(MIC) (or a derivativethereof), determine a magnitude of such signal, and output a peakamplitude signal PEAK indicative of such magnitude. Dynamic rangeenhancement controller 222 may include any suitable system, device, orapparatus configured to receive peak amplitude signal PEAK and basedthereon, determine an analog gain for audio system 200 and generate ananalog gain signal ANALOG_GAIN indicative of such analog gain. Analoggain signal ANALOG_GAIN may be used as a control signal in order to seta magnitude of AC bias voltage V_(BIAS). Accordingly, the magnitude ofAC bias voltage V_(BIAS) may be set based on the magnitude of soundpressure upon diaphragm 108 in order to maximize a dynamic range ofmicroelectromechanical systems microphone 100 and audio system 200. Forexample, to maximize dynamic range, dynamic range enhancement controller222 may cause the magnitude of AC bias voltage V_(BIAS) to increase asthe sound pressure upon diaphragm 108 decreases, and cause the magnitudeof AC bias voltage V_(BIAS) to decrease as the sound pressure upondiaphragm 108 increases.

Capacitance measurement subsystem 224 may include any suitable system,device, or apparatus configured to measure a capacitance of firstcapacitor 204 (as explicitly shown in FIG. 2) and/or second capacitor206 and output a gain compensation signal GAIN_COMP indicative of suchmeasured capacitance. Such capacitance may be indicative of thesensitivity of measurement of audio system 200, and thus, as describedbelow, may be used to compensate for such sensitivity.

Gain controller 226 may include any suitable system, device, orapparatus configured to receive analog gain signal ANALOG_GAIN and/orgain compensation signal GAIN_COMP and based thereon, generate a digitalgain signal DIG_GAIN, which may be used by gain element 228 to control adigital gain applied to digital microphone signal MIC_DIG to generate anaudio output signal AUDIO_OUT of audio system 200. Accordingly, digitalgain signal DIG_GAIN may include two components. First, the level ofdigital gain indicated by digital gain signal DIG_GAIN may be based onan inverse of analog gain indicated by analog gain signal ANALOG_GAIN,in order to offset the analog gain applied in audio system 200 (e.g., tovoltage supply 202) to maximize dynamic range. Second, the level ofdigital gain indicated by digital gain signal DIG_GAIN may also controla gain of audio system 200 responsive to the sensitivity of measurement,as may be indicated by the first capacitance of capacitor 204 and/or thesecond capacitance of capacitor 206.

Advantageously, the systems and methods herein may provide animprovement over existing MEMS microphone implementations. For example,replacing the direct-current (DC) biasing present in existing approacheswith the AC biasing approach of the systems and methods disclosed hereincreates a signal of interest modulated by a frequency of AC bias voltageV_(BIAS), reducing or eliminating flicker noise and/or bias resistornoise that is typically present at low frequencies or at DC. Inaddition, because of the symmetric excitation applied to the capacitorbridge of audio system 200, the magnitude of AC bias voltage V_(BIAS)may be modified without disturbing a bias point of the sensing amplifier(e.g., amplifier 212) of audio system 200. Accordingly, such magnitudeof AC bias voltage V_(BIAS) may be modified as a function of themagnitude of the acoustic pressure incident upon MEMS microphone 100,thus maximizing the dynamic range of MEMS microphone 100 and audiosystem 200. Furthermore, unlike DC-biased implementations, in which onlythe change in capacitance of MEMS microphone 100 can be measured but notthe static capacitance, in the systems and methods described herein,such static capacitance can be measured, allowing for collection ofadditional information regarding audio system 200 (e.g., sensitivity),for which audio system 200 may make further adjustments to improveaccuracy.

This disclosure encompasses all changes, substitutions, variations,alterations, and modifications to the example embodiments herein that aperson having ordinary skill in the art would comprehend. Similarly,where appropriate, the appended claims encompass all changes,substitutions, variations, alterations, and modifications to the exampleembodiments herein that a person having ordinary skill in the art wouldcomprehend. Moreover, reference in the appended claims to an apparatusor system or a component of an apparatus or system being adapted to,arranged to, capable of, configured to, enabled to, operable to, oroperative to perform a particular function encompasses that apparatus,system, or component, whether or not it or that particular function isactivated, turned on, or unlocked, as long as that apparatus, system, orcomponent is so adapted, arranged, capable, configured, enabled,operable, or operative.

All examples and conditional language recited herein are intended forpedagogical objects to aid the reader in understanding the disclosureand the concepts contributed by the inventor to furthering the art, andare construed as being without limitation to such specifically recitedexamples and conditions. Although embodiments of the present disclosurehave been described in detail, it should be understood that variouschanges, substitutions, and alterations could be made hereto withoutdeparting from the spirit and scope of the disclosure.

1. A microelectromechanical systems transducer, comprising: a backplatecomprising a first plurality of electrodes comprising at least a firstelectrode and a second electrode electrically isolated from one anotherand each is mechanically coupled to the backplate in a fixedrelationship relative to the backplate; and a diaphragm configured tomechanically displace relative to the backplate as a function ofpressure incident upon the diaphragm, wherein the diaphragm comprises asecond plurality of electrodes, the second plurality of electrodescomprising at least a third electrode and a fourth electrode, whereinthe third electrode and the fourth electrode are electrically isolatedfrom one another and each is mechanically coupled to the diaphragm in afixed relationship relative to the diaphragm such that the secondplurality of electrodes mechanically displace relative to the backplateas the function of pressure incident upon the diaphragm; wherein: thefirst electrode and the third electrode form a first capacitor having afirst capacitance which is a function of a displacement of the diaphragmrelative to the backplate; the second electrode and the fourth electrodeform a second capacitor having a second capacitance which is a functionof the displacement of the diaphragm relative to the backplate; and eachof the first capacitor and the second capacitor are biased by analternating-current voltage waveform; wherein the first electrode iselectrically coupled to a first terminal of a voltage supply providingthe alternating-current voltage waveform and the second electrode iselectrically coupled to a second terminal of the voltage supply. 2.(canceled)
 3. The microelectromechanical systems transducer of claim 1,wherein the first capacitor and the second capacitor are electricallycoupled to one another in a bridge structure, the bridge structurecomprising: a third capacitor in series with the first capacitor andcoupled between the first capacitor and the second terminal of thevoltage supply; and a fourth capacitor in series with the secondcapacitor and coupled between the second capacitor and the firstterminal of the voltage supply.
 4. The microelectromechanical systemstransducer of claim 3, further comprising a direct-current voltagedetector configured to detect a direct-current voltage offset present inthe bridge structure.
 5. The microelectromechanical systems transducerof claim 4, wherein the direct-current voltage detector is furtherconfigured to control at least one of a third capacitance of the thirdcapacitor and a fourth capacitance of the fourth capacitor responsive tothe direct-current voltage offset.
 6. The microelectromechanical systemstransducer of claim 1, wherein the alternating-current voltage waveformhas a frequency greater than that of human hearing.
 7. Themicroelectromechanical systems transducer of claim 1, wherein amagnitude of the alternating-current voltage waveform is set based on amagnitude of pressure upon the diaphragm in order to maximize a dynamicrange of the microelectromechanical systems microphone.
 8. Themicroelectromechanical systems transducer of claim 7, wherein themagnitude of the alternating-current voltage waveform increases as thepressure upon the diaphragm decreases and vice versa.
 9. Themicroelectromechanical systems transducer of claim 1, further comprisinga capacitance detector configured to measure at least one of the firstcapacitance and the second capacitance.
 10. The microelectromechanicalsystems transducer of claim 9, wherein the capacitance detector isfurther configured to control a gain of the microelectromechanicalsystems microphone responsive to the measurement of at least one of thefirst capacitance and the second capacitance.
 11. A method, comprising:biasing a first capacitor of an electromechanical systems transducerwith an alternating-current voltage waveform; biasing a second capacitorof the electromechanical systems transducer with the alternating-currentvoltage waveform; wherein: the first capacitor is formed from a firstelectrode and a third electrode; the second capacitor is formed from asecond electrode and a fourth electrode; the first electrode and thesecond electrode are electrically isolated from one another and each ismechanically coupled to a backplate in a fixed relationship relative tothe backplate, the backplate comprising a first plurality of electrodescomprising the first electrode and the second electrode; the thirdelectrode and the fourth electrode are electrically isolated from oneanother and each is mechanically coupled to a diaphragm in a fixedrelationship relative to the diaphragm such that the second plurality ofelectrodes mechanically displace relative to the backplate as thefunction of pressure incident upon the diaphragm which mechanicallydisplaces the diaphragm relative to the backplate; the first capacitorhas a first capacitance which is a function of a displacement of thediaphragm relative to the backplate; and the second capacitor has asecond capacitance which is a function of a displacement of thediaphragm relative to the backplate; and electrically coupling the firstelectrode to a first terminal of a voltage supply providing thealternating-current voltage waveform and electrically coupling thesecond electrode to a second terminal of the voltage supply. 12.(canceled)
 13. The method of claim 11, wherein the first capacitor andthe second capacitor are electrically coupled to one another in a bridgestructure, the bridge structure comprising: a third capacitor in serieswith the first capacitor and coupled between the first capacitor and thesecond terminal of the voltage supply; and a fourth capacitor in serieswith the second capacitor and coupled between the second capacitor andthe first terminal of the voltage supply.
 14. The method of claim 13,further comprising detecting a direct-current voltage offset present inthe bridge structure.
 15. The method of claim 14, further comprisingcontrolling at least one of a third capacitance of the third capacitorand a fourth capacitance of the fourth capacitor responsive to thedirect-current voltage offset.
 16. The method of claim 11, wherein thealternating-current voltage waveform has a frequency greater than thatof human hearing.
 17. The method of claim 11, further comprising settinga magnitude of the alternating-current voltage waveform based on amagnitude of pressure upon the diaphragm in order to maximize a dynamicrange of the microelectromechanical systems transducer.
 18. The methodof claim 17, further comprising increasing the magnitude of thealternating-current voltage waveform as the pressure upon the diaphragmdecreases and vice versa.
 19. The method of claim 11, further comprisingmeasuring at least one of the first capacitance and the secondcapacitance.
 20. The method of claim 19, further comprising controllinga gain of the microelectromechanical systems transducer responsive tothe measurement of at least one of the first capacitance and the secondcapacitance.